When a counterweight is attached to a crane in order to enable the crane to lift heavy loads, certain problems arise that have not heretofore been satisfactorily solved.
Ordinarily a crane to which a counterweight is applied comprises an earthborne base or understructure on which a platform or crane deck is mounted for swiveling about a vertical axis; and the platform, in turn, supports a generally upright boom which is swingable back and forth in a vertical plane about a horizontal axis that extends through or near the vertical swiveling axis. A load lifting line that passes over the upper end of the boom and normally depends from it is connected with a power driven winch on the platform.
Since the boom normally has a forward inclination, the lifting of a heavy load tends to tilt the crane forward, and a large enough load could tip it over. Counterweights in one form or another are often employed to resist these tilting forces.
The simplest form of counterweight is one that is rigidly attached to the platform, either on a rearwardly extending portion of the platform itself or on a rigid arm projecting rearwardly from it. The disadvantage of a rigid connection between a very heavy counterweight mass and the platform is that when the crane is unloaded, the counterweight imposes high tilting stresses upon the swiveling connection between the platform and the crane base.
To avoid this disadvantage, counterweight carriers on castered wheel carriages have been connected to the platform, to swing with the platform as it swivels and to ride back and forth with the entire crane when the crawler treads are driven. Because such a mobile counterweight is supported by the surface that its wheels engage, it imposes no tilting force on the swivel joint for the platform when the crane is unloaded.
U.S. Pat. No. 4,258,852, to D. C. Juergens, discloses counterweight carriers connected with the platform by rigid rearwardly extending arms that can swing up and down relative to the platform about a horizontal axis which passes close to the vertical swiveling axis. Through guy lines extending from the top end of the boom to the counterweight carriers, the boom is stabilized by the oppositely directed forces of the load and the counterweights. The counterweight carriers are at all times in contact with the surface underlying them, owing to the pivoted connection between the counterweight carrier arms and the platform, which allows the carriers to move up and down relative to the rest of the crane as they pass over small irregularities in the surface on which they ride. Hence, the counterweights do not impose any tilting force upon the swivel connection between the platform and the crane base at times when the crane is not supporting a load. However, this arrangement has the disadvantage that forward tilting forces on the boom are supported only by the counterweights, and therefore, the counterweight moment must always be large enough to offset the largest moment that will be imposed on the tip of the boom by a load to be lifted. A heavy counterweight and a long arm, needed for a very heavy load, impede and complicate the swiveling movements of the crane when it is hoisting a light load, and therefore this arrangement requires time consuming adjustments for adapting it to different loads.
U.S. Pat. No. 3,842,984, to Brown et al, discloses a counterweight carrier mounted on wheels and connected with the platform by means of arms that are swingable up and down, essentially as in the above described arrangement; but provision is made for adding the stabilizing forces of the crane base to those provided by the counterweight when the counterweight, by itself, would be too light to support tilting forces on the boom. In Brown et al, the boom projects from the platform at a forward inclination, and behind it is a mast that projects up from the platform at a rearward inclination. An adjustable line linkage connects the tips of the mast and the boom, and from the tip of the mast a second adjustable line linkage is connected to a gantry on the platform. The second linkage, when taut, can carry forward tilting force on the boom and the mast into the platform and the crane base, but normally it is slack and does not do so. Instead there is a third adjustable line linkage, connected between the tip of the mast and the counterweight carrier, whereby forward tilting force upon the boom is initially imposed upon the counterweight carrier. Thus, if the hoisted load is heavy enough, the counterweight carrier is lifted off of the ground. As it rises, allowing the mast to swing forward, the second linkage is tensioned, so that when the counterweight carrier has been lifted up off the ground to a certain extent the forward tilting forces generated by the load are imposed upon the platform through the mast and the second linkage. Obviously the success of this arrangement depends upon proper adjustment of the second and third linkages. If too much slack is left in the second linkage, the boom can swing too far forward as the counterweight rises and can reach a position at which the load moment at its tip will exceed the sum of the offsetting forces exerted by the counterweight and the crane base.
As explained in U.S. Pat. No. 3,842,984, it has also been proposed (U.S. Pat. No. 3,037,643) to mount a counterweight on a rearwardly projecting portion of the platform that is supported on a castered wheel arrangement. Although the counterweight is supported on its wheel assembly when the boom is not loaded, and at all times cooperates with the main chassis or crane base to resist tilting forces imposed by a load on the boom, the arrangement has the important disadvantage that all stresses due to irregularities in the surface that supports the counterweight wheel assembly will be imposed upon the swiveling connection between the platform and the crane base.